Fix type of msgpack packer callback.
Fix type of msgpack packer callback.

tudang/paxosudp,tyh20082000/libpaxos,tudang/paxosudp,tudang/paxosudp,tudang/paxosudp,tudang/paxosudp,tyh20082000/libpaxos,tudang/paxosudp,tyh20082000/libpaxos,tyh20082000/libpaxos